


What a good idea

by Sin_Cognito



Category: Undertale (Video Game)
Genre: Alternate Universe - Swapfell, Cat Ears, Cat Tail, M/M, Pet Play, Swapfell Papyrus - Freeform, Swapfell Sans, heatisiac
Language: English
Status: Completed
Published: 2018-01-28
Updated: 2018-01-28
Packaged: 2019-03-10 14:52:54
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 2,279
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/13503849
Author URL: https://archiveofourown.org/users/Sin_Cognito/pseuds/Sin_Cognito
Summary: 1st prize for a raffle from tumblr.Papyrus makes Sans drink heatisiac but the effects are surprising... Maybe Papyrus should have checked the label on the bottle first.





	What a good idea

**Author's Note:**

> It’s indeed swapfellcest, but the two are still called ‘Sans’ and ‘Papyrus’ because there’s no mention of other AUs.

Papyrus felt like the luckiest bastard under Earth.

First, of course, he was dating his Lord. This didn’t need further explainations. They were lovers in the face of all opposition and Queen Toriel knew there were a lot in their forsaken world.

The second reason had only occurred to him this morning, during an errand his Lord had commanded. As always, after he got every supply his Lord had asked, he went to the illegal stand behind Hotland. Just to get his regular dose of cigarettes. And maybe… a little extra for the bedroom…

His Lord often came home with all sorts of toys and drugs for them to further enjoy their sessions together. Papyrus was always on the receiving end of such accessories and boooy did he love every second of their plays. He even began to go buy those toys and other elixirs himself.

Today just so happened that he had a little more time before going home, so after greeting the two girl monsters managing the booth, he quickly glanced at the table for any new and interesting toy. His Lord had a considerable amount of dildos of various sizes and shapes so today he was more looking for something else, something unusual. Maybe a new lube, with an interesting flavor ? Or a kinky costume he could dress in to put on a show for his little Master ? He didn’t have something particular in mind and was open to any suggestion the girls might make.

Unfortunately, the two salesmonsters had already sold the vast majority of their stock. They explained to Papyrus they were waiting for a delivery that was expected by the end of the week. Too bad, Papyrus thought, but what could he do ? Well, now that he was in the mood for sex, he could always ask his Lord for regular sex, or if the other felt like it too maybe even a sweet session of delicious punishment… And as a last resort, he could jerk himself off.

Not feeling too disappointed despite not finding anything, he turned away to go home when a flash of light caught his attention. Locating the source of the sparkling, he found it was coming from the back of the booth. It seemed to be a little bottle in a dirty crate.

“What about this ?” He heard himself ask.

“This ? Like it’s not really for sale though.”

“Like, not for you.”

“Just tell me what it is,” Papyrus insisted, intrigued.

“Well, like, it’s heatisiac but-”

“I’ll take it,” and Papyrus slammed all the coins he had left from his errand, practically snatching the bottle from the claws of the crocodile monster.

As soon as he heard the word ‘heat’, he knew he had to get the drink.

As much as he loved being his little Lord’s sub most of the time, his absolute favorite moments with Sans were when the latter was in heat. In those rare times, Sans would show a more submissive behavior, making Papyrus lose his head in euphoria. So to acquire a bottle of the so rare heatisiac drink ? It was like a message from the skies telling him to take control of what he rightfully deserved.

The walk back home was quick and thankfully uninterrupted and as soon as Papyrus closed the door behind him, he swiftly took a sip of the drink. Not too much, he reminded himself. He made a beeline for the kitchen where his Lord was busy cooking his favorite dish. He had truly improved over the years. His enchiladas were now edible.

Knowing he’d probably be punished for his bold behavior later (and frankly getting impatient at the thought), he silently came to stand behind his Lord’s back and quickly grabbed his wrists between his larger hands. Before his Master had the time to even throw insults at him, he bent down and captured Sans’ mouth in a rough and heated kiss, pushing down all the heatisiac he hadn’t swallowed. Unable to breathe, Sans had no choice but to accept the offered liquid and swallowed every last drop of it before Papyrus finally released him, not without a last brush of his orange tongue on his brother’s broader one.

“WHAT. DO. YOU. THINK. YOU’RE. DOING.” Magic was beginning to flare in Sans’ eyesocket which was never a good sign. Thankfully, Papyrus knew just how to soothe his brother.

“I’m hungry !” He stated.

It cut short Sans’ growing anger and Papyrus heard him mumble a quiet 'sit down then’ before they were both sat at the table, two generous portions of burnt enchiladas in their plate. They quietly ate, chitchatting about today’s work until Papyrus saw the magic drink manifest its effects in a… surprising way, to say the least. He couldn’t stop staring at Sans throughout desert and even if the little skeleton usually enjoyed being the center of attention, Papyrus’ unashamed gaze was particularly troubling.

“WHAT ?” Sans asked, exasperated. “DO I HAVE SOMETHING ON MY FACE ?”

Papyrus couldn’t help a chortle.

“DON’T YOU DARE LAUGH AT YOUR LORD AND MASTER, YOU FILTHY MUTT !” Sans shouted before lowering his voice to the familiar commanding tone they both loved so much. “Listen to me, you piece of garbage. I’m going to go to the bedroom and expect you to be ready for a punishment for your unacceptable behavior in precisely 10 minutes. Did I make myself clear ?”

“Yes m'Lord,” Papyrus diligently answered. Good thing he was used to replying to his Lord promptly, or he would have let another snicker slip.

True to his word, Sans stomped the stairs to the bedroom, slaming the door dramatically behind him. Exhaling slowly to calm himself, Papyrus took out from his pocket the vial of heatisiac to read more closely the label. Maybe he should’ve done that before.

In the center of the label, the word 'Heatisiac’ was written in elegant burgundy and golden letters. An intricate design of intertwined ribbons and vines circled the letters, giving the label a delicate and elegant look, that quite frankly went very well with the content of the bottle. Written in tiny and plain grey letters were the words Papyrus had overlooked before :

heat-inducing beverage  
special feline monsters edition

Well, Papyrus had supposed it was something like that. Otherwise, why in the world would Sans have summoned cat ears ?! And when he had turned away from the table to go upstairs, Papyrus had noticed the beginning of the forming of a… a… a freaking tail !! How in the world was Papyrus supposed to stay calm and wait for 10 whole minutes before joining Sans upstairs to-

A screeching noise suddenly cut through the silence that had settled in the kitchen after Sans left. It sounded like the howl of a wounded and scared animal. Panic took over Papyrus. That wasn’t a normal screech. Not for a skeleton monster. Maybe that had been a bad idea to make Sans drink an illegal beverage with unknown effects on non-feline monsters… Yeah okay it had completely been a bad idea.

Papyrus rushed to their bedroom, finding it dark and hauntingly silent.

“S-sans ?” Papyrus tried.

“nya”

“Sans ? Is that you ?”

“Pa-papy… What did you… do to meow ?!”

Papyrus squinted at the bed but could only distinguish the heavy blanket rolled up in a ball… Following his instinct, he lifted a corner of the blanket and jerked back when a very strong scent hit his nasal hole. A very strong heat scent that sent a pulse of arousal directly to Papyrus’ groin. In one swift movement, Papyrus tore away the annoying blanket and revelled in the sight that was offered.

Sans was on his hands and knees, his pelvis thrown up in the air and wiggling restlessly. Two purple ecto-ears twitched in a mix of irritation and embarassment on the top of his cranium. But the best part of the display was undoubtly the way Sans’ summoned fluffy tail was wound around his leg, the end swaying back and forth in hypnotic patterns.

Papyrus climbed on the bed, immediately freeing his cock from the too tight confines of his pants, palming himself to the sight of Sans’ sweet and glissening pussy.

“Papyyy… S-stop staring and do somethi- Ah ! Mmmmh…”

Papyrus grabbed Sans’ legs and pulled him until he was face to face with the feline skeleton’s ecto-pussy, swiping his tongue everywhere to lick up all the delicious juices from the sensible magic.

“C'mere kitty kitty, and show Papy your sweet pussy~” Papyrus chanted inbetween two licks of his tongue.

“Nnngh… Papy don’t ! talk like -Ah !- that !” Sans visibly had a hard time focusing and his words came out without any of his usual spite. If anything, they even sounded pleading to Papyrus’ non-existant ears. Sans was clearly experiencing a sub heat, and it must have been infuriating to the dom in him, Papyrus distantly thought. Yet he wouldn’t swap their positions for anything in the world right now. He was so rarely the dominant one he just couldn’t let the occasion slip.

While his mouth was preoccupied with eating out his brother’s cunt, Papyrus’ hands traveled accross the tiny body under him, finding with awe that every bone was covered by a very thin layer of soft fur. It wasn’t by any mean fluffly, but it added a nice touch to the natural beauty that was Sans. Papyrus couldn’t seem to stop fondling Sans’ ribcage, intertwining his fingers with the bottom-most ribs, creating delicious and litteral meowls of pleasure from Sans. His control over himself nearly broke when he began pulling at the ribs, the torture finally getting Sans’ resolve to crumble and the little monster began actually purring.

Unable to wait, Papyrus quickly turned Sans’ head to kiss him voraciously while he entered the warm paradise that was his magic pussy, pushing all the way down in one go. With how wet Sans was, the friction felt delicious and soon Papyrus was pulling out only to slam back seconds later. Papyrus set a punishing pace, not caring about gentleness in the early stage of the other’s artificial heat. He took a hold of Sans’ fluffly tail and pulled it in time with his powerful thrusts, drawing even more cries of pleasure from his brother.

Under him, Papyrus felt Sans’ body starting to go tense, the sign of his orgasm drawing near making the tall skeleton lose his mind. He quickly pulled out to turn Sans around so that he was laying on his back and slammed his cock back in place. Papyrus took advantage of the new position to sink in even deeper and felt his own cock twitch as each thrust was ponctuated by little 'nyas’ coming from his tiny lover. His was so close but he wanted Sans to be the first one to go.

“Come on, cum for me, my little kitty in heat,” Papyrus whispered next to Sans’ fluffy ears.

Papyrus let his long fingers brush against Sans’ clit and the tiny monster instantly arched his back, letting out a deafening yowl. The feeling of Sans’ pussy clentching around his cock finally did it for Papyrus, who spilled his load inside Sans’ magic. He didn’t stop thrusting until he was thoroughly milked dry.

Papyrus then just dropped his head next to Sans’ trembling body, and they both caught their breath in silence for a moment. All the while Papyrus’ hands kept caressing innocently Sans’ downy bones, going up and down in soothing motions and Sans began purring again. Papyrus’ soul leapt in joy when he felt Sans’ head lean into the touch when his bigger hands began fondling the magic ears.

Reveling in the quiet post-sex calm, Papyrus had almost forgotten the effects of heatisiac on a monster, but was reminded that Sans was still in heat when he felt the other’s tiny body climb on his larger ribcage. Sans’ purring increased in intensity again.

“We’re not done yet,” Sans drawled.

 

* * *

 

Lucky for him, Papyrus had a higher endurance than his Lord, which proved to be very helpful when dealing with Sans’ heats, artificial or not. He generally was able to last long enough to give pleasure to his lover, though it didn’t mean that he wasn’t completely spent after such sessions.

Thus, after spending several hours helping Sans get rid of the artificial heat, Papyrus was utterly exhausted, not even daring to move one bit in fear of experiencing sore bones. At least, now he was sure Sans’ heat had subsided : his tail and ears were gone, and he didn’t purr anymore either. Papyrus had truly enjoyed their time together, but he would be lying if he said he wasn’t glad it was over.

At one point, his Master had woken up enough to ask him what had happened, and Papyrus had to spill the beans, explaining he had found a bottle of heatisiac and made him drink it without reading the label. He expected his Lord to be mad at him, insult him and maybe punish him (though for once he hoped he wouldn’t because his Master’s punishments always turned him on, and right now being turned on was the last thing he wanted), but he had only remained silent for a while, seemingly pondering the new information. Papyrus began getting nervous, a silent Master usually wasn’t a good thing. He was actually fearing the reprimands.

But then his Lord mumbled something that Papyrus didn’t quite catch.

“What did you say m'Lord ?”

“I SAID ! Did you.. use… all of it ?” Sans repeated himself, averting his eyes in embarrassment. “I-I mean… I wouldn’t mind… too much… doing this again some time !”

Maybe the whole thing hadn’t been such a bad idea after all ?

**Author's Note:**

> I don't like the title. I couldn't find one I liked. Whatever.


End file.
